Photochromism is a well known physical phenomenon that is observed with certain classes of chemical compounds. A detailed discussion of this phenomenon can be found in “Photochromism: Molecules and Systems”, Studies in Organic Chemistry 40, edited by H. Durr and H. Bouas-Laurent, Elsevier, 1990.
A number of substituted 2H-naphtho[1,2-b]pyrans are known to be capable of exerting a reversible photochromic effect, as described for example in International Patent Application Nos. WO 95/16215 and WO 00/18755, and U.S. Pat. Nos. 6,387,512 and 6,353,102. However, none of these 2H-naphtho[1,2-b]pyrans compounds are reported to have dichroic properties.
Passive photochromic devices, i.e. devices containing photochromic dyes whose absorbance depends only from the presence or absence of UV light, typically exhibit rather quick activation (coloration) but it generally takes several minutes or even tens of minutes to revert from the colored to the bleached state. This slow fading is a severe drawback for the user of photochromic glasses who has to them take off to have clear vision when leaving the sunlight and entering dimmer light conditions.
Therefore, there is a need for compounds exhibiting not only good photochromic properties, such as high absorption in the colored state, fast coloring and fading rates, but which also are capable of dichroism and light polarization when in a spatially ordered condition, for example when incorporated into liquid crystals or oriented polymer host materials.